This invention concerns polymers for waste water clarification. More particularly, this invention is directed to cationic latex terpolymer flocculants and their use for sludge dewatering.
The dewatering of sewage sludges and similar organic suspensions, may be augmented by mixing into them chemical reagents in order to induce a state of coagulation or flocculation which thereby facilitates the process of separation of water. For this purpose, lime or salts of iron or aluminum have been utilized. More recently, synthetic polyelectrolytes, particularly certain cationic copolymers of acrylamide have been found to be useful.
Notwithstanding the variety of commercially available polymers which have been found to be capable of flocculating or coagulating organic sludges, there are various circumstances which tend to limit the usefulness of these reagents. Thus, while for certain sludges economical treatments with these known reagents are feasible, more often sludges require very high and cost-ineffective dosages of reagents in order to treat them successfully. Moreover, variations often occur in sludge from any one source. For example, variations in the supply of material to the sludge making process and/or in the oxidizing conditions that may be involved in the production of the sludge lead to a variety of particle types which must be removed. Furthermore, it is not uncommon to encounter sludges which are, for some reason, not amenable to flocculation by any of the known polymeric flocculating agents. It is therefore an object of the invention to provide to the art a superior method for the dewatering of sludge-containing waste waters.
A cationic water-in-oil emulsion polymer of acrylamide and a cationic monomer incorporating from about 0.1 to 0.75 mole percent of a hydroxyalkyl (meth)acrylate and from about 0.01 to 0.075 weight percent of a chain transfer agent for dewatering sludge is disclosed in U.S. Pat. No. 5,093,413. However, these polymers are relatively inefficient for dewatering sludge under high shear conditions such as centrifuge dewatering. Therefore, another object of this invention is to provide a new class of polymers that provide superior dewatering performance under high shear conditions.
We have discovered that cationic latex terpolymers incorporating from about 0.9 to about 5 mole percent hydroxyalkyl (meth)acrylate momomer(s) provide superior sludge dewatering performance, particularly under high shear conditions.
Accordingly, in its principal aspect, this invention is directed to a cationic latex terpolymer prepared by polymerizing from about 1 to about 99.1 mole percent of one or more cationic monomers, from about 0.9 to about 5 mole percent of one or more hydroxyalkyl (meth)acrylates and from 0 to about 98.1 mole percent of one or more nonionic monomers.
In another aspect, this invention is directed to a method of dewatering sludge comprising adding to the sludge an effective amount of a a cationic latex terpolymer prepared by polymerizing from about 1 to about 99.1 mole percent of one or more cationic monomers, from about 0.9 to about 5 mole percent of one or more hydroxyalkyl (meth)acrylates and from 0 to about 98.1 mole percent of one or more nonionic monomers.
In another aspect, this invention is directed to a polymer composition comprising a fluorescent tracer compound and a cationic latex terpolymer prepared by polymerizing from about 1 to about 99.1 mole percent of one or more cationic monomers, from about 0.9 to about 5 mole percent of one or more hydroxyalkyl (meth)acrylates and from 0 to about 98.1 mole percent of one or more nonionic monomers.
Definitions of Terms
xe2x80x9cAlkylxe2x80x9d means a monovalent group derived from a straight or branched chain saturated hydrocarbon by the removal of a single hydrogen atom. Representative alkyl groups include methyl, ethyl, n- and iso-propyl, and the like.
xe2x80x9cAlkylenexe2x80x9d means a divalent group derived from a straight or branched chain saturated hydrocarbon by the removal of two hydrogen atoms. Representative alkylene groups include methylene, ethylene, propylene, and the like.
xe2x80x9cBased on polymer activexe2x80x9d and xe2x80x9cbased on monomerxe2x80x9d mean the amount of a reagent added based on the level of vinylic monomer in the formula, or the level of polymer formed after polymerization, assuming 100% conversion.
xe2x80x9cBased on formulaxe2x80x9d means the amount of reagent added based on the total formula weight.
xe2x80x9cCationic Monomerxe2x80x9d means a monomer as defined herein which possesses a net positive charge. Representative cationic monomers include dialkylaminoalkyl acrylates and methacrylates and their quaternary or acid salts, including, but not limited to, dimethylaminoethyl acrylate methyl chloride quaternary salt, dimethylaminoethyl acrylate methyl sulfate quaternary salt, dimethyaminoethyl acrylate benzyl chloride quaternary salt, dimethylaminoethyl acrylate sulfuric acid salt, dimethylaminoethyl acrylate hydrochloric acid salt, dimethylaminoethyl methacrylate methyl chloride quaternary salt, dimethylaminoethyl methacrylate methyl sulfate quaternary salt, dimethylaminoethyl methacrylate benzyl chloride quaternary salt, dimethylaminoethyl methacrylate sulfuric acid salt, dimethylaminoethyl methacrylate hydrochloric acid salt, dialkylaminoalkylacrylamides or methacrylamides and their quaternary or acid salts such as acrylamidopropyltrimethylammonium chloride, dimethylaminopropylacrylamide methyl sulfate quaternary salt, dimethylaminopropylacrylamide sulfuric acid salt, dimethylaminopropylacrylamide hydrochloric acid salt, methacrylamidopropyltrimethylammonium chloride, dimethylaminopropylmethacrylamide methyl sulfate quaternary salt, dimethylaminopropylmethacrylamide sulfuric acid salt, dimethylaminopropylmethacrylamide hydrochloric acid salt, diethylaminoethyl acrylate, diethylaminoethyl methacrylate, diallyldiethylammonium chloride and diallyldimethyl ammonium chloride. Alkyl groups are generally C1-4 alkyl. Preferred cationic monomers are dimethylaminoethyl acrylate methyl chloride quaternaryl salt, dimethylaminoethyl methacrylate methyl chloride quaternary salt, acrylamidopropyltrimethylammonium chloride and methacrylamidopropyltrimethylammonium chloride. Dimethylaminoethyl acrylate methyl chloride quaternary salt is more preferred.
xe2x80x9cChain Transfer Agentxe2x80x9d means any molecule, used in free-radical polymerization, which will react with a polymer radical forming a dead polymer and a new radical. Representative Chain Transfer Agents are listed by K. C. Berger and G. Brandrup, xe2x80x9cTransfer Constants to Monomer, Polymer, Catalyst, Solvent, and Additive in Free Radical Polymerization,xe2x80x9d Section II, pp. 81-151, in xe2x80x9cPolymer Handbook,xe2x80x9d edited by J. Brandrup and E. H. Immergut, 3d edition, 1989, John Wiley and Sons, New York. Preferred chain transfer agents include sodium formate, 2-mercaptoethanol and isopropanol. Sodium formate is more preferred.
xe2x80x9cHydroxyalkyl (meth)acrylatexe2x80x9d means a compound of formula 
where R1 is H or CH3 and L is C1-C8, preferably C1-C4 alkylene. Representative hydroxyalkyl (meth)acrylates include hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, 6-hydroxyhexyl methacrylate, and the like. 2-Hydroxyethyl methacrylate (HEMA) and 2-hydroxypropyl methacrylate are preferred.
xe2x80x9cLatex polymerxe2x80x9d means a water-in-oil polymer emulsion comprising a cationic terpolymer according to this invention in the aqueous phase, a hydrocarbon oil for the oil phase and one or more water-in-oil emulsifying agents. Latex polymers are hydrocarbon continuous with the water-soluble polymers dispersed within the hydrocarbon matrix. The latex polymer is xe2x80x9cinvertedxe2x80x9d or activated for use by releasing the polymer from the particles using shear, dilution, and, generally, another surfactant. See U.S. Pat. No. 3,734,873, incorporated herein by reference. Representative preparations of high molecular weight inverse emulsion polymers are described U.S. Pat. Nos. 2,982,749; 3,284,393; and 3,734,873. See also, xe2x80x9cMechanism, Kinetics and Modeling of the Inverse-Microsuspension Homopolymerization of Acrylamide,xe2x80x9d Hunkeler, et al., Polymer (1989), 30(1), 127-42; and xe2x80x9cMechanism, Kinetics and Modeling of Inverse-Microsuspension Polymerization: 2. Copolymerization of Acrylamide with Quaternary Ammonium Cationic Monomers,xe2x80x9d Hunkeler et al., Polymer (1991), 32(14), 2626-40.
Inverse emulsion polymers are prepared by dissolving the desired monomers and any polymerization additives such as inorganic salts, chelants, pH buffers, and the like in the aqueous phase, dissolving the emulsifying agent(s) in the oil phase, emulsifying the water phase in the oil phase to prepare a water-in-oil emulsion, in some cases, homogenizing the water-in-oil emulsion, polymerizing the monomers dissolved in the water phase of the water-in-oil emulsion to obtain the polymer as a water-in-oil emulsion. If so desired, a self-inverting surfactant can be added after the polymerization is complete in order to obtain the water-in-oil self-inverting emulsion.
The oil phase comprises any inert hydrophobic liquid. Preferred hydrophobic liquids include aliphatic and aromatic hydrocarbon liquids including benzene, xylene, toluene, paraffin oil, mineral spirits, kerosene, naphtha, and the like. A paraffinic oil is preferred.
Free radical yielding initiators such as benzoyl peroxide, lauroyl peroxide, 2,2xe2x80x2-azobis (isobutyronitrile) (AIBN), 2,2xe2x80x2-azobis(2,4-dimethylvaleronitrile) (AIVN), potassium persulfate and the like are useful in polymerizing vinyl and acrylic monomers. 2,2xe2x80x2-azobis(isobutyronitrile) (AIBN) and 2,2xe2x80x2-azobis(2,4-dimethylvaleronitrile) (AIVN) are preferred. The initiator is utilized in amounts ranging between about 0.002 and about 0.2 percent by weight of the monomers, depending upon the solubility of the initiator.
Water-in-oil emulsifying agents useful for preparing the latex polymers of this invention include sorbitan esters of fatty acids, ethoxylated sorbitan esters of fatty acids, and the like or mixtures thereof. Preferred emulsifying agents include sorbitan monooleate, polyoxyethylene sorbitan monostearate, and the like. Additional details on these agents may be found in McCutcheon""s Detergents and Emulsifiers, North American Edition, 1980. Any inverting surfactant or inverting surfactant mixture described in the prior art may be used. Representative inverting surfactants include ethoxylated nonylphenol, ethoxylated linear alcohols, and the like. Preferred inverting surfactants are ethoxylated linear alcohols.
The polymer is prepared by polymerizing the appropriate monomers at a temperature of from about 30xc2x0 C. to about 85xc2x0 C. over about 1 to about 24 hours, preferably at a temperature of from about 40xc2x0 C. to about 70xc2x0 C. over about 3 to about 6 hours. Upon completion of the reaction, the water-in-oil emulsion polymer is cooled to room temperature, where any desired post-polymerization additives, such as antioxidants, or a high HLB surfactant (as described in U.S. Pat. No. 3,734,873) may be added.
The resulting emulsion polymer is a free-flowing liquid. An aqueous solution of the water-in-oil emulsion polymer can be generated by adding a desired amount of the emulsion polymer to water with vigorous mixing in the presence of a high-HLB surfactant (as described in U.S. Pat. No. 3,734,873).
xe2x80x9cMonomerxe2x80x9d means a polymerizable allylic, vinylic or acrylic compound. The monomer may be cationic or nonionic. Vinyl monomers are preferred, acrylic monomers are more preferred.
xe2x80x9cNonionic monomerxe2x80x9d means a monomer as defined herein which is electrically neutral. Representative non-ionic, water-soluble monomers include acrylamide, methacrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, N-isopropylacrylamide, N-vinylformamide, N-vinylmethylacetamide, N-vinyl pyrrolidone, N-t-butylacrylamide, N-methylolacrylamide, and the like. Preferred nonionic monomers are acrylamide and methacrylamide. Acrylamide is more preferred.
xe2x80x9cRSVxe2x80x9d stands for Reduced Specific Viscosity. The RSV of a polymer solution is a measure of the capacity of polymer molecules to enhance the viscosity of the solution at a given concentration, which depends on the structure of the polymer molecules (including size and shape), and interaction between polymer molecules. Within a series of polymer homologs which are substantially linear and well solvated, xe2x80x9creduced specific viscosity (RSV)xe2x80x9d measurements for dilute polymer solutions are an indication of polymer chain length and average molecular weight according to Paul J. Flory, in xe2x80x9cPrinciples of Polymer Chemistryxe2x80x9d, Cornell University Press, Ithaca, N.Y., 1953, Chapter VII, xe2x80x9cDetermination of Molecular Weightsxe2x80x9d, pp. 266-316. The RSV is measured at a given polymer concentration and temperature and calculated as follows:   RSV  =            [                        (                      η                          η              o                                )                -        1            ]        c  
wherein xcex7=viscosity of polymer solution;
xcex7o=viscosity of solvent at the same temperature; and
c=concentration of polymer in solution.
The units of concentration xe2x80x9ccxe2x80x9d are (grams/100 ml or g/deciliter). Therefore, the units of RSV are dL/g. In this patent application, for measuring RSV, the solvent used is 1.0 molar sodium nitrate solution. The polymer concentration in this solvent is 0.045 g/dL. The RSV is measured at 30xc2x0 C. The viscosities xcex7 and xcex7o are measured using a Cannon Ubbelohde semimicro dilution viscometer, size 75. The viscometer is mounted in a perfectly vertical position in a constant temperature bath adjusted to 30xc2x10.02xc2x0 C. The error inherent in the calculation of RSV is about 2 dl/grams. When two polymer homologs within a series have similar RSV""s that is an indication that they have similar molecular weights.
Preferred Embodiments
In a preferred aspect of this invention, the nonionic monomers are selected from acrylamide and methacrylamide and the cationic monomers are selected from dimethylaminoethyl acrylate methyl chloride quaternary salt, dimethylaminoethyl methacrylate methyl chloride quaternary salt, acrylamidopropyltrimethylammonium chloride and methacrylamidopropyltrimethylammonium chloride.
In another preferred aspect, the hydroxyalkyl (meth)acrylate is selected from hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate and 6-hydroxyhexyl methacrylate.
In another preferred aspect, the cationic latex polymer is prepared by polymerizing from about 20 to about 80 mole percent of one or more cationic monomers, from about 1 to about 2.5 mole percent of one or more hydroxyalkyl (meth)acrylates and from 17.5 to about 79 mole percent of one or more nonionic monomers.
In another preferred aspect, the nonionic monomer is acrylamide and the cationic monomer is dimethylaminoethyl acrylate methyl chloride quaternary salt.
In another preferred aspect, the hydroxyalkyl (meth)acrylate is hydroxyethyl methacrylate.
Sludges suitable for treatment using the polymers of this invention include primary, waste activated and aerobically and anaerobically digested industrial and municipal biosolids.
The total amount of polymer required to effectively flocculate the sludge may vary considerably according to the characteristics of the sludge being treated and the degree of dewatering required. Typically, the polymer is added in an amount of from about 10 ppm to about 600 ppm, preferably from about 15 ppm to about 400 ppm more preferably from about 20 ppm to about 200 ppm based on polymer actives.
Addition may be by conventional methods. Some agitation of the mixture of sludge and flocculent may be necessary to bring about flocculation. Thereafter separation of the separated solids from liquid may be effected by conventional methods, such as filtration and/or sedimentation.
In a preferred aspect of this invention, the sludge is dewatered by a high shear process.
In another preferred aspect, the high shear process is centrifuge sludge dewatering.
In centrifuge sludge dewatering, solids-liquid separation occurs in a centrifuge by rotating the sludge at high speeds to cause separation by gravitational forces. The gravitational force achieved in the centrifuge is in the range of 2000-3000 G. The solid bowl centrifuge is the type most often used for dewatering sludges. Solid bowl centrifuges are continuous flow-through systems.
The conical-cylindrical design is the most commonly used solid bowl centrifuge. It is a more flexible machine and can shift the balance of cake dryness and centrate quality over a broader range, depending upon the desired performance criteria. The conical-cylindrical solid bowl centrifuge consists of a rotating unit comprising a bowl and a conveyor joined through a special system of gears, which cause the bowl and conveyor to rotate in the same direction, but at slightly different speeds. The conical section at one end of the bowl forms a dewatering beach over which the conveyor pushes the sludge to outlet ports. The clarified supernatant liquid is allowed to escape over weirs or is removed by a skimmer.
The flocculated sludge upon entering the bowl is immediately subjected to not only the gravitational force, but also, to high impact shear arising from the sludge hitting the bowl wall. The sludge then travels to the conveyor section and the solids and liquid are separated. The shear involved in the centrifuge is different from that experienced in other dewatering devices such as belt filter press. In the latter, the water is initially allowed to drain by gravity after which the sludge is squeezed under pressure by the belts to release the extra water. Due to the high impact shear in the centrifuge, the flocs tend to break down rapidly and need polymers than can impart high floc strength. As shown below, the cationic terpolymers of this invention provide flocs of high shear strength compared to the prior art cationic polymers.
The performance of the cationic terpolymers of this invention may be monitored by means of an inert fluorescent tracer as described in U.S. Pat. No. 4,783,314, incorporated herein by reference. In particular, a composition comprising a cationic latex terpolymer according to this invention and an inert fluorescent tracer compound in a known ratio is added to the sludge being treated as described above. The fluorescent emission of the treated sludge is measured and used to quantify and control the amount and feed rate of the polymer to achieve maximum dewatering performance.
xe2x80x9cInert fluorescent tracer compoundxe2x80x9d means a material which is capable of fluorescing while present in the sludge being treated. The inert fluorescent tracer compound should not be appreciably affected by any other material present in the sludge, or by the temperature or temperature changes encountered during the dewatering process. Representative inert fluorescent tracer compounds include mono-, di-, and trisulfonated naphthalenes and their water soluble salts, sulfonated derivatives of pyrene and their water soluble salts such as 1,3,6,8-pyrenetetrasulfonic acid, and Acid Yellow 7. A preferred inert fluorescent tracer compound is 1,3,6,8-pyrenetetrasulfonic acid, sodium salt.
The cationic latex terpolymer/inert fluorescent tracer compound composition is prepared by adding the inert fluorescent tracer compound with stirring to the cationic latex terpolymer of this invention. An inverting surfactant as described herein may be added along with the inert fluorescent tracer compound. The amount of inert fluorescent tracer compound added may be readily determined by one of ordinary skill in the art, taking into consideration the polymer composition and the characteristics of the sludge being treated.
The foregoing may be better understood by reference to the following Examples, which are presented for purposes of illustration and are not intended to limit the scope of this invention.